The light will come on if either one of those problems happen.
If it's full, then you need to have it checked.
You could have a leak, a problem with the master cylinder, the proportioning valve.
Brakes are not something you want to gamble with.
It's one thing to throw your fan belt and be stranded on the side of the road, it's another to run a stop sign and crash into a bus load of nuns, taking kittens to an orphanage.